Ones more aimed at social sciences (that I am aware of) are;
- Social Science Research Network (SSRN)
- The National Bureau of Economic Research (NBER)
- The recently-founded SocArXiv (link to search and upload preprints)
SSRN is dominated by economic and legal research (and NBER is obviously focused on economics).
Another I recently became aware of is Academia.edu, although this appears to me more like a personal website that has the option to upload working papers than an organized repository like SSRN or arXiv (here is an example profile page on Academia.edu).